This invention relates to polymers useful as a viscosity index improver and to oil compositions compromising the same. More particularly, this invention relates to hydrogenated copolymers of isoprene and a monoalkenyl aromatic hydrocarbon.
As is well known, the viscosity of lubricating oils varies with temperature, and it is important that the oil not be too viscous (thick) at low temperature nor too fluid (thin) at higher temperature. The variation in the viscosity-temperature relationship of an oil is indicated by the so-called viscosity index (VI). The higher the viscosity index, the less the change in viscosity with temperature. In general, the viscosity index is a function of the oils viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature for lubricating oils have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C.
The thickening efficiency of a polymeric VI improver is an important, and frequently, the principal consideration in its selection for use in oil compositions. Polymeric VI improvers which significantly increase the high temperature kinematic viscosity without appreciably increasing the low temperature kinematic viscosity are preferred. The thickening efficiency of any given polymeric VI improver will vary with polymer composition and structure but will increase with increased molecular weight. Other properties are important including the ability of the VI improver to maintain an increase in viscosity even after subjected to mechanical shear; the high temperature, high shear rate (HTHSR) viscosity response of an oil composition containing the viscosity index improver; the low temperature viscosity response of an oil containing the viscosity index improver; the engine pumpability of a lubricating oil composition containing the viscosity index improver; and the low temperature startability of an engine containing the lubricating oil composition. It should be noted that viscosity index improvers which are solid and do not exhibit cold flow are particularly preferred for ease in packaging and handling. Polymers of this type are usually, but not always, capable of being separated from solvent during manufacture by means of cyclone-finishing techniques.
Block copolymers comprising a single polymeric block of a monoalkenyl aromatic hydrocarbon compound and a single polymeric block of isoprene offer a good balance of viscosity index improver properties as taught in U.S. Pat. No. 3,772,196. However, these polymers comprise relatively high monoalkenyl aromatic hydrocarbon contents which give lower HTHSR responses than polymers having similar molecular weights and less of the monoalkenyl aromatic hydrocarbons.
A viscosity index improver having a relatively small amount of monoalkenyl aromatic hydrocarbon is taught in U.S. Pat. No. 3,775,329 which describes tapered copolymers of isoprene and monoalkenyl aromatic hydrocarbon monomers. Although the patent excludes the use of randomizers, it is asserted that the tapered copolymers do not have significant homopolymer blocks of isoprene or the monoalkenyl aromatic hydrocarbons. The described VI improvers include non-cyclone finishable VI improvers, VI improvers which provide relatively low HTHSR responses in multigrade oils, and VI improvers with low thickening efficiencies.
U.S. Pat. No. 4,418,234 describes diene block copolymers which have high vinyl contents and provide good high shear rate responses. For any molecular weight, an increase in vinyl content lowers the thickening efficiency of the VI improver. The patent asserts that randomizers can be used as long as the total amount of homopolymer blocks of monoalkenyl aromatic hydrocarbon is less than about 5 percent by weight, preferably less than about 2 percent.